The load carrying capacity of a truck trailer is determined by the number of axles it has, in other words on a “per axle” basis. In some instances carrying capacity of a trailer is enhanced by adding an intermediate axle assembly between the tractor wheelset and the rear wheelset of the trailer.
A challenge with adding an intermediate axle assembly is in steering the trailer when the wheels associated with the intermediate axle assembly are in contact with the underlying ground. One solution to the problem is to raise the intermediate axle assembly to lift the associated wheels off of the ground in turns. A problem with such an approach is that the weight carried by the intermediate axle assembly gets transferred to the remaining axles which may overload and damage any underlying pavement.
An alternative solution to the steering issue is to provide the intermediate axle with steering capabilities. Such is referred to as a “steer axle”. A typical steer axle comprises a laterally extending beam having respective spindles mounted to opposite ends thereof. The spindles are in effect stub axles and carry the intermediate wheels. The spindles are carried by yokes which in turn are swivelably connected to the beam by respective kingpin assemblies.
Each kingpin assembly has a kingpin which is rigidly secured in a generally vertical orientation to each end of the beam so as to protrude above and below the beam. The yoke has upper and lower sleeves which receive the kingpin above and below the beam respectively. The upper and lower sleeves are spaced apart and span the portion of the beam surrounding the kingpin.
It will be appreciated that the yoke and the kingpin assembly is subject to a considerable load. The lower sleeve of the yoke supports the overlying portion of the end of the beam and therefore the weight of anything supported by the beam. To reduce friction and wear in this area a bronze thrust washer is interspersed therebetween which in turn encircles the kingpin. The thrust washer has facility for greasing but by its nature does not hold grease well as its faces press directly against the thrust faces of the components adjacent thereto. Hence the thrust washer requires regular and frequent maintenance for any longevity and even with such treatment it tends to be relatively short lived. The yoke to kingpin securement is typically effected using bronze bushings which are pressed into the ends of the sleeves and which are a close fit to the kingpins. The nature of bronze bushings is that they require frequent lubrication absent which their service life is very short and even with frequent lubrication the bushing life is relatively short.
It is an object of the present invention to provide a steer axle assembly which overcomes at least some of the above maintenance and wear problems.